Transversely-excited film bulk acoustic resonator package

ABSTRACT

Acoustic resonator devices and filters are disclosed. An acoustic resonator chip includes a piezoelectric plate attached to a substrate. Portions of the piezoelectric plate form at least first and second diaphragms spanning respective cavities in the substrate. A first conductor pattern on the surface of the piezoelectric plate includes a first plurality of contact pads and at least first and second IDTs with interleaved fingers of each IDT on respective diaphragms. An interposer includes a second plurality of contacts pads. A plurality of conductive balls bond each of the contact pads of the first plurality of contact pads to respective contact pads of the second plurality of contact pads.

RELATED APPLICATION INFORMATION

The patent is a continuation of copending application Ser. No.17/082,945, titled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATORPACKAGE, filed Oct. 28, 2020, which is a continuation-in-part ofapplication Ser. No. 16/998,300 titled TRANSVERSELY-EXCITED FILM BULKACOUSTIC RESONATOR, filed Aug. 20, 2020, which is a divisional of U.S.application Ser. No. 16/841,134 titled TRANSVERSELY-EXCITED FILM BULKACOUSTIC RESONATOR PACKAGE AND METHOD, filed Apr. 6, 2020, now U.S. Pat.No. 10,819,309 granted Oct. 27, 2020 which claims priority to thefollowing provisional patent applications: application 62/830,258,titled XBAR PACKAGING, filed Apr. 5, 2019; application 62/881,749,titled XBAR PACKAGING INCLUDING CAP PLATE, filed Aug. 1, 2019; andapplication 62/904,416, titled XBAR WAFER-LEVEL PACKAGING, filed Sep.23, 2019, all of which are incorporated herein by reference in theirentirety.

Application Ser. No. 17/082,945 is also a continuation-in-part ofcopending U.S. application Ser. No. 16/920,173 titledTRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR, filed Jul. 2, 2020,which is a continuation of application Ser. No. 16/438,121 titledTRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR, filed Jun. 11, 2019,now U.S. Pat. No. 10,756,697, which is a continuation-in-part ofapplication Ser. No. 16/230,443, entitled TRANSVERSELY-EXCITED FILM BULKACOUSTIC RESONATOR, filed Dec. 21, 2018, now U.S. Pat. No. 10,491,192,which claims priority from the following provisional patentapplications: application 62/685,825, filed Jun. 15, 2018, entitledSHEAR-MODE FBAR (XBAR); application 62/701,363, filed Jul. 20, 2018,entitled SHEAR-MODE FBAR (XBAR); application 62/741,702, filed Oct. 5,2018, entitled 5 GHZ LATERALLY-EXCITED BULK WAVE RESONATOR (XBAR);application 62/748,883, filed Oct. 22, 2018, entitled SHEAR-MODE FILMBULK ACOUSTIC RESONATOR; and application 62/753,815, filed Oct. 31,2018, entitled LITHIUM TANTALATE SHEAR-MODE FILM BULK ACOUSTICRESONATOR. All of these applications are incorporated herein byreference.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic waveresonators, and specifically to filters for use in communicationsequipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a pass-band or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is better than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. The currentLTE™ (Long Term Evolution) specification defines frequency bands from3.3 GHz to 5.9 GHz. These bands are not presently used. Future proposalsfor wireless communications include millimeter wave communication bandswith frequencies up to 28 GHz.

For example, the peak speed for fourth generation (“4G”) cellularservice is typically 100 megabits per second for high mobilitycommunication (such as from trains and cars) and 1 gigabit per second(Gbit/s) for low mobility communication (such as pedestrians andstationary users). However, the new fifth generation (“5G”) networkswill need to have greater bandwidth, giving higher download speeds,eventually up to 10-20 gigabits per second (Gbit/s).

In order to handle the increased bandwidth of 5G cellular service; and 5GHz and 6 GHz WiFi, wide bandwidth filters will be needed for wirelessfrequencies above 3 GHz. For example, the 5G bands n77 and n79 usefrequency bands at 3.3-4.2 GHz and 4.4-5.0 GHz, respectively. Thus,there will be a need for RF filters that not only operate at higherfrequency but also have a much wider bandwidth than those currentlyavailable.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave (BAW) resonators, film bulkacoustic wave resonators (FBAR), and other types of acoustic resonators.However, these existing technologies are not well-suited for use at thehigher frequencies proposed for future communications networks.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view and two schematic cross-sectionalviews of a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 2 is an expanded schematic cross-sectional view of a portion of theXBAR of FIG. 1.

FIG. 3A is an alternative schematic cross-sectional view of the XBAR ofFIG. 1.

FIG. 3B is a graphic illustrating a shear horizontal acoustic mode in anXBAR.

FIG. 4A is a schematic cross-sectional view of a packaged XBAR.

FIG. 4B is a schematic cross-sectional view of another packaged XBAR.

FIG. 5A is a graph of the transmission S21 through a bandpass filterusing XBARs with a distance between the XBARs and a silicon cover as aparameter.

FIG. 5B is a schematic block diagram of a filter using XBARs.

FIG. 6A is a schematic cross-sectional view of an XBAR filter chip andan interposer prior to bonding.

FIG. 6B is a schematic cross-sectional view of a packaged XBAR filter.

FIG. 7A is a schematic cross-sectional view of another packaged XBARfilter.

FIG. 7B is a schematic cross-sectional view of another packaged XBARfilter.

FIG. 8 is a schematic cross-sectional view of another packaged XBARfilter.

FIG. 9 is a schematic cross-sectional view of another packaged XBARfilter.

FIG. 10 is a schematic cross-sectional view of another packaged XBARfilter.

FIG. 11 is a flow chart of a process for fabricating an XBAR chip.

FIG. 12A, FIG. 12B, and FIG. 12C are, in combination, a flow chart ofprocess for packaging an XBAR filter.

FIG. 13 is a flow chart of another process for packaging an XBAR filter.

FIG. 14 is a flow chart of another process for packaging an XBAR filter.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

FIG. 1 shows a simplified schematic top view and orthogonalcross-sectional views of a transversely-excited film bulk acousticresonator (XBAR) 100. XBAR resonators such as the resonator 100 may beused in a variety of RF filters including band-reject filters, band-passfilters, duplexers, and multiplexers. XBARs are particularly suited foruse in filters for communications bands with frequencies above 3 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on asurface of a piezoelectric plate 110 having parallel front and backsurfaces 112, 114, respectively. The piezoelectric plate is a thinsingle-crystal layer of a piezoelectric material such as lithiumniobate, lithium tantalate, lanthanum gallium silicate, gallium nitride,or aluminum nitride. The piezoelectric plate is cut such that theorientation of the X, Y, and Z crystalline axes with respect to thefront and back surfaces is known and consistent. In the examplespresented in this patent, the piezoelectric plates are Z-cut, which isto say the Z axis is normal to the surfaces. However, XBARs may befabricated on piezoelectric plates with other crystallographicorientations.

The back surface 114 of the piezoelectric plate 110 is attached to asubstrate 120 that provides mechanical support to the piezoelectricplate 110. The substrate 120 may be, for example, silicon, sapphire,quartz, or some other material. The back surface 114 of thepiezoelectric plate 110 may be bonded to the substrate 120 using a waferbonding process, or grown on the substrate 120, or attached to thesubstrate in some other manner. The piezoelectric plate may be attacheddirectly to the substrate, or may be attached to the substrate via oneor more intermediate material layers.

The conductor pattern of the XBAR 100 includes an interdigitaltransducer (IDT) 130. The IDT 130 includes a first plurality of parallelfingers, such as finger 136, extending from a first busbar 132 and asecond plurality of fingers extending from a second busbar 134. Thefirst and second pluralities of parallel fingers are interleaved. Theinterleaved fingers overlap for a distance AP, commonly referred to asthe “aperture” of the IDT. The center-to-center distance L between theoutermost fingers of the IDT 130 is the “length” of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR100. A radio frequency or microwave signal applied between the twobusbars 132, 134 of the IDT 130 excites a primary acoustic mode withinthe piezoelectric plate 110. As will be discussed in further detail, theexcited primary acoustic mode is a bulk shear mode where acoustic energypropagates along a direction substantially orthogonal to the surface ofthe piezoelectric plate 110, which is also normal, or transverse, to thedirection of the electric field created by the IDT fingers. Thus, theXBAR is considered a transversely-excited film bulk wave resonator.

A cavity 140 is formed in the substrate 120 such that a portion 115 ofthe piezoelectric plate 110 containing the IDT 130 is suspended over thecavity 140 without contacting the substrate 120. “Cavity” has itsconventional meaning of “an empty space within a solid body.” The cavity140 may be a hole completely through the substrate 120 (as shown inSection A-A and Section B-B) or a recess in the substrate 120 (as shownsubsequently in FIG. 3A). The cavity 140 may be formed, for example, byselective etching of the substrate 120 before or after the piezoelectricplate 110 and the substrate 120 are attached. As shown in FIG. 1, thecavity 140 has a rectangular shape with an extent greater than theaperture AP and length L of the IDT 130. A cavity of an XBAR may have adifferent shape, such as a regular or irregular polygon. The cavity ofan XBAR may more or fewer than four sides, which may be straight orcurved.

The portion 115 of the piezoelectric plate suspended over the cavity 140will be referred to herein as the “diaphragm” (for lack of a betterterm) due to its physical resemblance to the diaphragm of a microphone.The diaphragm may be continuously and seamlessly connected to the restof the piezoelectric plate 110 around all, or nearly all, of perimeterof the cavity 140. In this context, “contiguous” means “continuouslyconnected without any intervening item”.

For ease of presentation in FIG. 1, the geometric pitch and width of theIDT fingers is greatly exaggerated with respect to the length (dimensionL) and aperture (dimension AP) of the XBAR. A typical XBAR has more thanten parallel fingers in the IDT 110. An XBAR may have hundreds, possiblythousands, of parallel fingers in the IDT 110. Similarly, the thicknessof the fingers in the cross-sectional views is greatly exaggerated.

FIG. 2 shows a detailed schematic cross-sectional view of the XBAR 100of FIG. 1. The piezoelectric plate 110 is a single-crystal layer ofpiezoelectrical material having a thickness ts. ts may be, for example,100 nm to 1500 nm. When used in filters for LTE™ bands from 3.4 GHZ to 6GHz (e.g. bands 42, 43, 46), the thickness ts may be, for example, 200nm to 1000 nm.

A front-side dielectric layer 214 may optionally be formed on the frontside of the piezoelectric plate 110. The “front side” of the XBAR is, bydefinition, the surface facing away from the substrate. The front-sidedielectric layer 214 has a thickness tfd. The front-side dielectriclayer 214 is formed between the IDT fingers 238. Although not shown inFIG. 2, the front side dielectric layer 214 may also be deposited overthe IDT fingers 238. A back-side dielectric layer 216 may optionally beformed on the back side of the piezoelectric plate 110. The back-sidedielectric layer 216 has a thickness tbd. The front-side and back-sidedielectric layers 214, 216 may be a non-piezoelectric dielectricmaterial, such as silicon dioxide or silicon nitride. tfd and tbd maybe, for example, 0 to 500 nm. tfd and tbd are typically less than thethickness is of the piezoelectric plate. tfd and tbd are not necessarilyequal, and the front-side and back-side dielectric layers 214, 216 arenot necessarily the same material. Either or both of the front-side andback-side dielectric layers 214, 216 may be formed of multiple layers oftwo or more materials.

The front side dielectric layer 214 may be formed over the IDTs of some(e.g., selected ones) of the XBAR devices in a filter. The front sidedielectric 214 may be formed between and cover the IDT finger of someXBAR devices but not be formed on other XBAR devices. For example, afront side dielectric layer may be formed over the IDTs of shuntresonators to lower the resonance frequencies of the shunt resonatorswith respect to the resonance frequencies of series resonators, whichhave thinner or no front side dielectric. Some filters may include twoor more different thicknesses of front side dielectric over variousresonators. The resonance frequency of the resonators can be set thus“tuning” the resonator, at least in part, by selecting a thicknesses ofthe front side dielectric.

Further, a passivation layer may be formed over the entire surface ofthe XBAR device 100 except for contact pads where electric connectionsare made to circuitry external to the XBAR device. The passivation layeris a thin dielectric layer intended to seal and protect the surfaces ofthe XBAR device while the XBAR device is incorporated into a package.The front side dielectric layer and/or the passivation layer may be,SiO₂, Si₃N₄, Al₂O₃, some other dielectric material, or a combination ofthese materials.

The thickness of the passivation layer may be selected to protect thepiezoelectric plate and the metal electrodes from water and chemicalcorrosion, particularly for power durability purposes. It may range from10 to 100 nm. The passivation material may consist of multiple oxideand/or nitride coatings such as SiO2 and Si3N4 material.

The IDT fingers 238 may be aluminum or a substantially aluminum alloy,copper or a substantially copper alloy, beryllium, gold, or some otherconductive material. Thin (relative to the total thickness of theconductors) layers of other metals, such as chromium or titanium, may beformed under and/or over the fingers to improve adhesion between thefingers and the piezoelectric plate 110 and/or to passivate orencapsulate the fingers. The busbars (132, 134 in FIG. 1) of the IDT maybe made of the same or different materials as the fingers.

Dimension p is the center-to-center spacing or “pitch” of the IDTfingers, which may be referred to as the pitch of the IDT and/or thepitch of the XBAR. Dimension w is the width or “mark” of the IDTfingers. The IDT of an XBAR differs substantially from the IDTs used insurface acoustic wave (SAW) resonators. In a SAW resonator, the pitch ofthe IDT is one-half of the acoustic wavelength at the resonancefrequency. Additionally, the mark-to-pitch ratio of a SAW resonator IDTis typically close to 0.5 (i.e. the mark or finger width is aboutone-fourth of the acoustic wavelength at resonance). In an XBAR, thepitch p of the IDT is typically 2 to 20 times the width w of thefingers. In addition, the pitch p of the IDT is typically 2 to 20 timesthe thickness is of the piezoelectric slab 212. The width of the IDTfingers in an XBAR is not constrained to one-fourth of the acousticwavelength at resonance. For example, the width of XBAR IDT fingers maybe 500 nm or greater, such that the IDT can be fabricated using opticallithography. The thickness tm of the IDT fingers may be from 100 nm toabout equal to the width w. The thickness of the busbars (132, 134 inFIG. 1) of the IDT may be the same as, or greater than, the thickness tmof the IDT fingers.

FIG. 3A is an alternative cross-sectional view along the section planeA-A defined in FIG. 1. In FIG. 3A, a piezoelectric plate 310 is attachedto a substrate 320. A portion of the piezoelectric plate 310 forms adiaphragm 315 spanning a cavity 340 in the substrate. The cavity 340,does not fully penetrate the substrate 320, and is formed in thesubstrate under the portion of the piezoelectric plate 310 containingthe IDT of an XBAR. The cavity 340 may be formed, for example, byetching the substrate 320 before attaching the piezoelectric plate 310.Alternatively, the cavity 340 may be formed by etching the substrate 320with a selective etchant that reaches the substrate through one or moreopenings 342 provided in the piezoelectric plate 310. In this case, thediaphragm 315 may contiguous with the rest of the piezoelectric plate310 around a large portion of a perimeter 345 of the cavity 340. Forexample, the diaphragm 315 may contiguous with the rest of thepiezoelectric plate 310 around at least 50% of the perimeter of thecavity 340.

The XBAR 300 shown in FIG. 3A will be referred to herein as a“front-side etch” configuration since the cavity 340 is etched from thefront side of the substrate 320 (before or after attaching thepiezoelectric plate 310). The XBAR 100 of FIG. 1 will be referred toherein as a “back-side etch” configuration since the cavity 140 isetched from the back side of the substrate 120 after attaching thepiezoelectric plate 110.

The XBARs of FIG. 1 and FIG. 3A and filter devices using XBARs must beencased in a package. The package for an XBAR must perform the followingfunctions:

-   -   Provide mechanical protection for the diaphragms and conductor        patterns;    -   Provide cavities facing the diaphragms comparable to the cavity        340 in the substrate 320;    -   Provide a seal to prevent intrusion of humidity and/or fluids        that may be encountered during subsequent assembly of the        packaged filter into an electronic device; and    -   Provide means for connecting the conductor patterns of the XBARs        to circuitry external to the packaged filter device.

FIG. 3B is a graphical illustration of the primary acoustic mode ofinterest in an XBAR. FIG. 3B shows a small portion of an XBAR 350including a piezoelectric plate 360 and three interleaved IDT fingers380. An RF voltage is applied to the interleaved fingers 380. Thisvoltage creates a time-varying electric field between the fingers. Thedirection of the electric field is lateral, or parallel to the surfaceof the piezoelectric plate 360, as indicated by the arrows labeled“electric field”. Due to the high dielectric constant of thepiezoelectric plate, the electric field is highly concentrated in theplate relative to the air. The lateral electric field introduces sheardeformation, and thus strongly excites a primary shear-mode acousticmode, in the piezoelectric plate 360. In this context, “sheardeformation” is defined as deformation in which parallel planes in amaterial remain parallel and maintain a constant distance whiletranslating relative to each other. A “shear acoustic mode” is definedas an acoustic vibration mode in a medium that results in sheardeformation of the medium. The shear deformations in the XBAR 350 arerepresented by the curves 390, with the adjacent small arrows providinga schematic indication of the direction and magnitude of atomic motion.The degree of atomic motion, as well as the thickness of thepiezoelectric plate 360, have been greatly exaggerated for ease ofvisualization. While the atomic motions are predominantly lateral (i.e.horizontal as shown in FIG. 3B4), the direction of acoustic energy flowof the excited primary shear acoustic mode is substantially orthogonalto the front and back surface of the piezoelectric plate, as indicatedby the arrow 395.

Considering FIG. 3B, there is essentially no electric field immediatelyunder the IDT fingers 380, and thus acoustic modes are only minimallyexcited in the regions 397 under the fingers. There may be evanescentacoustic motions in these regions. Since acoustic vibrations are notexcited under the IDT fingers 380, the acoustic energy coupled to theIDT fingers 380 is low (for example compared to the fingers of an IDT ina SAW resonator), which minimizes viscous losses in the IDT fingers.

An acoustic resonator based on shear acoustic wave resonances canachieve better performance than current state-of-the artfilm-bulk-acoustic-resonators (FBAR) and solidly-mounted-resonatorbulk-acoustic-wave (SMR BAW) devices where the electric field is appliedin the thickness direction. In such devices, the acoustic mode iscompressive with atomic motions and the direction of acoustic energyflow in the thickness direction. In addition, the piezoelectric couplingfor shear wave XBAR resonances can be high (>20%) compared to otheracoustic resonators. High piezoelectric coupling enables the design andimplementation of microwave and millimeter-wave filters with appreciablebandwidth.

FIG. 4A and FIG. 4B are schematic cross-sectional views of packaged XBARfilters, each of which comprises an XBAR filter chip 405 and aninterposer 450. While the cross-sectional views of FIG. 4A and FIG. 4Bshow the XBAR filter chips 405 containing two XBARs, filters maycommonly include five to nine XBARs. Specifically, FIG. 4A is aschematic cross-sectional view of a packaged filter 400A usingfront-side etched XBARs, and FIG. 4B is a schematic cross-section viewof a packaged XBAR filter 400B using back-side etched XBARs. FIG. 4A andFIG. 4B are intended to illustrate the requirements on the package foran XBAR filter but do not necessarily represent practical packagestructures.

Referring to FIG. 4A and FIG. 4B, the XBAR filter chip 405 includes apiezoelectric plate 410 attached to a substrate 420. Portions of thepiezoelectric plate form diaphragms spanning respective cavities 440 inthe substrate 420. Commonly, one or more intermediate layers, which areshown in FIG. 4A and FIG. 4B but not identified, may be disposed betweenthe piezoelectric plate 410 and the substrate 420. Conductor patternsincluding IDTs are formed on the surface of the piezoelectric plate 410such that interleaved IDT fingers, such as fingers 430, are disposed onthe diaphragms.

The interposer 450 also includes a base 452 and conductive vias 454 toprovide electrical connections between the conductor patters on thepiezoelectric plate 410 and circuitry 490 external to the filter, suchas using solder bumps 495. The term “interposer” is generally used todescribe a passive circuit device that provides electrical connectionsbetween two different interfaces, such as contacts of the chip 405 andcontacts of the circuitry 490. The interposer 450 fulfills this functionbut also forms a structural part of the package of the packaged XBARfilter 400A. The interposer may be, for example, a printed circuit board(PCB), a low temperature cofired ceramic (LTCC) circuit card, a siliconwafer, or some other structure that provides mechanical protection tothe diaphragms of the XBARs.

In FIG. 4A, the conductive vias 454 are illustrated schematically assimple pins extending though the base to the piezoelectric plate. Aswill be discuss subsequently, physically realizable vias have morecomplex structures.

As shown in FIG. 4A, the interposer 450 includes recesses 455 facing thediaphragms of the XBARs. Such recesses may be required to ensuresufficient spacing between the diaphragms and the surfaces of theinterposer facing the diaphragms (i.e. the bottom surfaces of thecavities 440). The required spacing (dimension cd in the FIG. 4A)depends on the material of the interposer.

Acoustic RF filters usually incorporate multiple acoustic resonators.Typically, these resonators have at least two different resonancefrequencies. For example, an RF filter using the well-known “ladder”filter architecture includes shunt resonators and series resonators. Ashunt resonator typically has a resonance frequency below the passbandof the filter and an anti-resonance frequency within the passband. Aseries resonator typically has a resonance frequency within the passband and an anti-resonance frequency above the passband. In manyfilters, each resonator has a unique resonance frequency. An ability toobtain different resonance frequencies for XBARs made on the samepiezoelectric plate greatly simplifies the design and fabrication of RFfilters using XBARs.

FIG. 5A is a plot of the transmission (S21) through a bandpass filterincluding five XBARs in a package including a base made from a highresistivity silicon wafer. The silicon wafer has high dielectricpermittivity and finite conductivity that can distort the electricfields created by the IDTs within the XBARs and introduce electricallosses, and thus impact the filter performance. The solid line 510 is aplot of S12 for a filter where the dimension cd is 50 microns. Thedashed line 520 is a plot of S12 for the same filter where the dimensioncd is 15 microns. The dotted line 530 is a plot of S12 for the samefilter where the dimension cd is 5 microns. Reducing the spacing betweenthe surface of the interposer and the diaphragms from 50 microns to 15microns reduces the transmission of the filter in the passband by about0.5 dB. Further reducing the spacing between the surface of theinterposer and the diaphragms to 5 microns results in an additionreduction in transmission by about 1.0 dB. 15 microns may be a practicalminimum for the dimension cd. Increasing the spacing between the surfaceof the interposer beyond 50 microns, for example to 100 microns, mayoffer a small addition improvement in transmission. All transmissionplots are based on simulations of packaged filters using finite elementmethods.

FIG. 5B is a schematic circuit diagram and layout for a high frequencyband-pass filter 550 using XBARs. The filter 550 has a conventionalladder filter architecture including three series resonators 560A, 560B,560C and two shunt resonators 570A, 570B. The three series resonators560A, 560B, and 560C are connected in series between a first port and asecond port. In FIG. 5B, the first and second ports are labeled “In” and“Out”, respectively. However, the filter 550 is bidirectional and eitherport and serve as the input or output of the filter. The two shuntresonators 570A, 570B are connected from nodes between the seriesresonators to ground. All the shunt resonators and series resonators areXBARs.

The three series resonators 560A, B, C and the two shunt resonators570A, B of the filter 550 are formed on a single plate 580 ofpiezoelectric material bonded to a silicon substrate (not visible). Eachresonator includes a respective IDT (not shown), with at least thefingers of the IDT disposed over a cavity in the substrate. In this andsimilar contexts, the term “respective” means “relating things each toeach”, which is to say with a one-to-one correspondence. In FIG. 5B, thecavities are illustrated schematically as the dashed rectangles (such asthe rectangle 585). In this example, each IDT is disposed over arespective cavity. In other filters, the IDTs of two or more resonatorsmay be disposed over a single cavity.

In some cases, the IDTs include at least five IDTs, each IDT having arecess in the interposer facing the diaphragm and a distance from abottom of the recess to the diaphragm that is greater than or equal to15 microns and less than or equal to 100 microns. The first conductorpattern includes electrical connections that connect the shuntresonators and the series resonators in a bandpass ladder filterarchitecture, such as shown in FIG. 5B.

Two or more portions of the piezoelectric plate each may form at leasttwo diaphragms, each diaphragm having an IDT and spanning a respectivecavity. In some cases, the two or more portions of the piezoelectricplate are portions of a single piezoelectric plate that spans all of thecavities. In other cases, the two or more portions of the piezoelectricplate are two separate pieces of piezoelectric plate and are separatedby an etched trench through the piezoelectric plate. Here, the trenchmay be etched by patterning all of the plate except where trenches aredesired between and to separate or dice each diaphragm from all others.This may be done prior to or after mounting the plate(s) on thesubstrate. The patterning may use a photoresist as described herein. Theetch may be a wet or dry etch such as an etch used to etch the conductormaterial as described herein.

Referring back to FIG. 4A, the interposer 450 is attached to thepiezoelectric plate 410 by a seal 460. The seal 460 provides mechanicalattachment and prevents intrusion of humidity and other fluids into theinterior of the packaged XBAR filter 400A. As shown in FIG. 4A, the seal460 is a distinct structure having a finite thickness which contributesto the total spacing cd between the diaphragms and the facing surfacesof the interposer 450. The seal 460 may be, for example, athermocompression or ultrasonic bond between metal layers deposited onthe piezoelectric plate and the interposer, a polymer or adhesive bond,a eutectic or solder bond, a glass frit bond, or some other bondingmethod and structure. Alternatively, the seal may be a bond, such as aplasma activated or surface activated wafer bond, directly between theinterposer 450 and the piezoelectric plate 410. In this case (not shownin FIG. 4A), the thickness of the seal 460 may be negligible. In allcases, the seal 460 is present around the entire perimeter of thepackaged XBAR filter 400A. In addition, the same sealing mechanism mayattach the piezoelectric plate 410 to the interposer 450 at locations,such as location 465, in the interior of the packaged XBAR filter 400A.

FIG. 4B is a schematic cross-section view of a packaged XBAR filter 400Busing back-side etched XBARs. Except for the depth of the cavities 440,the structure of the XBAR filter chip 405, the interposer 450, and theseal 460 between the interposer and the piezoelectric plate areidentical to the comparable elements in FIG. 4A. The descriptions ofthese elements will not be repeated.

In addition, the package XBAR filter 400B includes a cap 480 attached tothe substrate 420 by a cap seal 485. The cap 480 may be any materialsuitable to cover the openings where the cavities 440 intersect thesurface of the substrate 420. For example, the cap 480 may be silicon,glass, quartz, or a polymer plate or film. The cap seal 485 may be anyof the materials and sealing methods previously described with respectto the seal 460.

FIG. 4B also illustrates the packaged XBAR filter 400B attached andelectrically connected to a radio module circuit board 490 by means ofsolder balls 495. This is an example of the use of the packaged filterdevice. The radio module circuit board 490 and the solder balls 495 arenot part of the filter device 400B.

FIG. 6A is an exploded schematic cross-sectional view of a packaged XBARfilter 600. More specifically, FIG. 6A shows schematic cross-sectionalviews of an XBAR filter chip 605 including front-side etched XBARs andan interposer 650 prior to bonding.

The XBAR filter chip 605 includes a piezoelectric plate 610 attached toa substrate 620. The substrate 620 may be high resistivity silicon orsome other material. Portions of the piezoelectric plate 610 formdiaphragms spanning respective cavities 640 in the substrate 620.Commonly, one or more intermediate layers, which are shown in FIG. 6Abut not identified, may be disposed between the piezoelectric plate 610and the substrate 620. A first conductor pattern is formed on thesurface of the piezoelectric plate 610. The first conductor patternincludes IDTs with interleaved IDT fingers, such as fingers 630,disposed on the diaphragms. The first conductor pattern may be aluminum,copper, molybdenum, or some other metal with a thickness of about 100 nmto 1000 nm.

A second conductor pattern is formed on the surface of the piezoelectricplate 610. The second conductor pattern, which may overlay portions ofthe first conductor pattern, may be gold, aluminum, copper or some othermetal. The second conductor pattern includes a continuous metal ring 662around the perimeter of the XBAR filter chip 605. The second conductorpattern also includes pads, such as pad 672, in locations where portionsof the first conductor pattern must be connected to circuitry externalto the packaged XBAR filter.

The interposer 650 includes a base 652, which may be high resistivitysilicon or some other material. The base 652 may have recesses 655 sothat the surfaces of the base 652 facing the diaphragms (i.e. thebottoms of the recesses 655) are sufficiently far from the diaphragms. Athird conductor pattern is formed on the surface of the base 652 facingthe XBAR filter chip 605. The third conductor pattern may be the samematerial as the second conductor pattern. The third conductor patternincludes a continuous metal ring 664 around the perimeter of the base652. The third conductor pattern also includes pads, such as pad 674, inlocations where portions of the first conductor pattern must beconnected to circuitry external to the packaged XBAR filter. Thearrangement of ring 664 and pads 674 of third conductor pattern istypically a mirror image of the arrangement of the ring 662 and pads 672of the second conductor pattern. Pads that are arranged in a mirrorimage may be described as respective, matching or opposing pads of thetwo conductive patterns.

The interposer 650 also includes vias such as via 676. When the base issilicon, such vias are commonly referred to as “through silicon vias”(TSVs). Vias consist of a metal-coated or metal-filled hole through thebase 652. Each via provides an electrical connection between one of thepads, such as pad 674, of the third conductor pattern and acorresponding pad on the external surface (i.e. the lower surface asshown in the figure) of the base 652. While FIG. 6A, which is intendedto illustrate the structure of the packaged XBAR filter 600, shows theTSVs formed prior to bonding the XBAR filter chip 605 and the interposer650, the vias may be formed after bonding.

FIG. 6B is a schematic cross-sectional view of the packaged XBAR filter600 after the XBAR filter chip 605 and the interposer 650 of FIG. 6A arebonded to each other. Descriptions of all of the identified elements inFIG. 6B were previously provided in the discussion of FIG. 6A and willnot be repeated.

As shown in FIG. 6B, the ring 662 around the perimeter of the XBARfilter chip 605 has been directly bonded to the ring 664 around theperimeter of the interposer 650 to create a hermetic seal around theperimeter of the package XBAR filter 600. In this context, the term“directly bonded” means bonded without any intervening adhesive.Simultaneously, the pads, such as pad 672, of the second conductorpattern have been directly bonded to the pads, such as pad 674, of thethird conductor pattern to create electrical connections between theXBAR filter chip 605 and the interposer 650. The bonds between the ringsand pads of the second and third conductor patterns may be accomplishedby, for example, thermo-compression bonding or ultrasonic bonding.

FIG. 7A is a schematic cross-sectional view of a packaged XBAR filter700A including an XBAR filter chip 705 bonded to an interposer 750. Withthe exception of elements 780 and 785, the identified elements in FIG.7A have the structure and function as the corresponding elements of FIG.6A and FIG. 6B. Descriptions of these elements will not be repeated.

A cap 780 is sealed to the substrate 720 by a cap seal 785. When thesubstrate 720 of the XBAR filter chip 705 and the base 752 of theinterposer 750 are both silicon, the cap 780 may also be silicon tomaintain consistency of thermal expansion coefficients. In other cases,the cap 780 may be silicon, borosilicate or other glass, plastic, orsome other material. The cap 780 may be attached to the substrate 720using any of the previously described sealing methods and materials.Typically, the cap 780 will be attached to the substrate 720 immediatelyafter forming the cavities 740 in the substrate. The cap 780 may beattached to the substrate 720 before bonding the XBAR filter chip 705 tothe interposer 750.

FIG. 7B is a schematic cross-sectional view of a packaged XBAR filter700B including an XBAR filter chip 705 bonded to an interposer 750. Withthe exception of element 768, the identified elements in FIG. 7B havethe structure and function as the corresponding elements of FIG. 6A andFIG. 6B. Descriptions of these elements will not be repeated.

In the packaged XBAR filter 700B, a perimeter seal between thepiezoelectric plate 710 and the base 752 is not made by bondingconductor rings (i.e. conductor rings 762, 772 in FIG. 7A). Instead, aring of cured adhesive material 768 forms a perimeter seal between thepiezoelectric plate 710 and the base 752. The cured adhesive material768 may be, for example, an epoxy resin or other thermosetting adhesive.The adhesive material (in an uncured state) may be applied to either orboth of the piezoelectric plate 710 and the base 752 before thepiezoelectric plate 710 and the base 752 are assembled. The adhesivematerial may be cured after or concurrent with bonding the pads 772 tothe pads 774.

The XBAR filter chips 605 and 705 shown in FIG. 6B, FIG. 7A, and FIG. 7Bmay be portions of large wafers containing many filter chips. Similarly,the interposers 650 and 750 may be portions of large wafers containing acorresponding number on interposers. An XBAR wafer and an interposerwafer may be bonded and individual packaged XBAR filters may be excisedfrom the bonded wafers.

FIG. 8 is a schematic cross-sectional view of another packaged XBARfilter 800 including an XBAR filter chip 805 with front-side etchedcavities 840 and a low temperature cofired ceramic (LTCC) interposer850. As in the previous examples, the XBAR filter chip 805 includes apiezoelectric plate 810 attached to a substrate 820. The substrate 820may be high resistivity silicon or some other material. Portions of thepiezoelectric plate 810 form diaphragms spanning respective cavities 840in the substrate 820. Commonly, one or more intermediate layers, whichare shown in FIG. 8 but not identified, may be disposed between thepiezoelectric plate 810 and the substrate 820. A conductor pattern isformed on the surface of the piezoelectric plate 810. The conductorpattern includes IDTs with interleaved IDT fingers, such as fingers 830,disposed on the diaphragms.

The LTCC interposer 850 comprises layers of thin ceramic tape, some orall of which bear printed conductors, that are assembled and then firedto form a rigid multilayer circuit board. In the example of FIG. 8, theinterposer has three conductor layers 874, 876, 878. An LTCC interposerfor an XBAR filter may have more than three layers. The availability ofmultiple conductor layers allows incorporation of passive components,such as inductors, into the interposer. The interposer 850 may also haveconductive vias similar to vias 454 extending through each layer toelectrically connect contact pads formed on the interposer back surfaceto contact pads formed on the interposer front surface. The conductivevias through one layer may be connected by the printed conductors to thevias through another layer. The conductive vias and printed conductorscan connect the contact pads of the interposer back surface to contactpads formed on the interposer front surface.

The LTCC interposer 850 may have recesses 855 to ensure sufficientspacing between the diaphragms and the surfaces of the interposer facingthe diaphragms. Such recess may be formed, for example, by punchingopenings in one or more of the ceramic layers prior to cofiring thelayers of the interposer.

The XBAR filter chip 805 is flip-chip mounted to the interposer 850.Flip-chip mounting establishes physical and electric connections betweenthe XBAR filter chip 805 and the interposer 850. As shown in FIG. 8, theconnections are made by means of solder balls such as solder ball 872.Alternatively, the connections made be made by thermocompression orultrasonic bonding of gold bumps on the XBAR filter chip 805 and theinterposer 850 (not shown).

Since flip-chip mounting does not establish a seal between the XBARfilter chip 805 and the interposer 850, a polymer cover 860 is moldedover the assembly to provide a near-hermetic seal.

FIG. 9 is a schematic cross-sectional view of another packaged XBARfilter 900 including an XBAR filter chip 905 with back-side etchedcavities and a low temperature cofired ceramic (LTCC) interposer 950.With the exception of element 980, the identified elements in FIG. 9have the structure and function as the corresponding elements of FIG. 8.Descriptions of these elements will not be repeated.

A cap 980 is sealed to the substrate 920. Since the cap 980 iseventually enclosed by the molded cover 970, the cap's primary functionis to prevent intrusion of materials, including the molding compoundused for the cover 970, into the cavities 940. This function may besatisfied by a very thin cap, such as a plastic film.

FIG. 10 is a schematic cross-sectional view of another packaged XBARfilter 1000 including an XBAR filter chip 1005 with back-side etchedcavities and an interposer 1050 formed by layers built up on the surfaceof the XBAR filter chip. The XBAR filter chip 1005 is a portion of awafer (not shown) containing multiple XBAR filter chips. The build up ofthe interposer layers is done on all of the XBAR filter chipssimultaneously. Individual packaged XBAR filters are then excised fromthe wafer.

As in previous examples, the XBAR filter chip 1005 includes apiezoelectric plate 1010 attached to a substrate 1020. The substrate1020 may be high resistivity silicon or some other material. Portions ofthe piezoelectric plate 1010 form diaphragms spanning respectivecavities 1040 in the substrate 1020. Commonly, one or more intermediatelayers, which are shown in FIG. 10 but not identified, may be disposedbetween the piezoelectric plate 1010 and the substrate 1020. A conductorpattern is formed on the surface of the piezoelectric plate 1010. Theconductor pattern includes IDTs with interleaved IDT fingers, such asfingers 1030, disposed on the diaphragms. A cap 1080 is sealed to thesubstrate 1020 by a cap seal 1085 as previously described.

The interposer 1050 includes at least three layers sequentially formedon the piezoelectric plate 1010. Walls 1052 surround the diaphragms ofthe XBAR devices. The thickness of the walls 1052 defines the distancebetween the diaphragms and a cover layer 1054 that spans the wallscreating an enclosed cavity 1055 over each diaphragm. Both the walls1052 and the cover layer 1054 may be polymer materials. An interposerconductor pattern 1070 includes pads 1072 on the external surface of thecover layer 1054 for connection to circuitry external to the packagedXBAR filter. The conductor pattern 1070 connects the pads 1072 toconnection points 1074 on the XBAR filer chip 1005. The conductorpattern 1070 may be aluminum, copper, gold, or a combination ofmaterials.

Description of Methods

FIG. 11 is a simplified flow chart showing a process 1100 for making anXBAR filter chip. The process 1100 starts at 1105 with a substrate and aplate of piezoelectric material and ends at 1195 with a completed XBARfilter chip. The flow chart of FIG. 11 includes only major processsteps. Various conventional process steps (e.g. surface preparation,cleaning, inspection, baking, annealing, monitoring, testing, etc.) maybe performed before, between, after, and during the steps shown in FIG.11.

The flow chart of FIG. 11 captures three variations of the process 1100for making an XBAR filter chip which differ in when and how cavities areformed in the substrate. The cavities may be formed at steps 1110A,1110B, or 1110C. Only one of these steps is performed in each of thethree variations of the process 1100.

The piezoelectric plate may be, for example, Z-cut lithium niobate orlithium tantalate as used in the previously presented examples. Thepiezoelectric plate may be some other material and/or some other cut.The substrate may preferably be silicon. The substrate may be some othermaterial that allows formation of deep cavities by etching or otherprocessing.

In one variation of the process 1100, one or more cavities are formed inthe substrate at 1110A, before the piezoelectric plate is bonded to thesubstrate at 1120. A separate cavity may be formed for each resonator ina filter device. The one or more cavities may be formed usingconventional photolithographic and etching techniques. Typically, thecavities formed at 1110A will not penetrate through the substrate, andthe resulting resonator devices will have a cross-section as shown inFIG. 3A or FIG. 3B.

At 1120, the piezoelectric plate is bonded to the substrate. Thepiezoelectric plate and the substrate may be bonded by a wafer bondingprocess. Typically, the mating surfaces of the substrate and thepiezoelectric plate are highly polished. One or more layers ofintermediate materials, such as an oxide or metal, may be formed ordeposited on the mating surface of one or both of the piezoelectricplate and the substrate. One or both mating surfaces may be activatedusing, for example, a plasma process. The mating surfaces may then bepressed together with considerable force to establish molecular bondsbetween the piezoelectric plate and the substrate or intermediatematerial layers.

A conductor pattern, including IDTs of each XBAR, is formed at 1130 bydepositing and patterning one or more conductor layer on the front sideof the piezoelectric plate. The conductor layer may be, for example,aluminum, an aluminum alloy, copper, a copper alloy, or some otherconductive metal. Optionally, one or more layers of other materials maybe disposed below (i.e. between the conductor layer and thepiezoelectric plate) and/or on top of the conductor layer. For example,a thin film of titanium, chrome, or other metal may be used to improvethe adhesion between the conductor layer and the piezoelectric plate. Aconduction enhancement layer of gold, aluminum, copper or other higherconductivity metal may be formed over portions of the conductor pattern(for example the IDT bus bars and interconnections between the IDTs). Insome cases, the conductor pattern includes electrical connections thatconnect the shunt resonators and the series resonators in a bandpassladder filter architecture, such as shown in FIG. 5B.

The conductor pattern may be formed at 1130 by depositing the conductorlayer and, optionally, one or more other metal layers in sequence overthe surface of the piezoelectric plate. The excess metal may then beremoved by etching through patterned photoresist. The conductor layercan be etched, for example, by plasma etching, reactive ion etching, wetchemical etching, and other etching techniques.

Alternatively, the conductor pattern may be formed at 1130 using alift-off process. Photoresist may be deposited over the piezoelectricplate. and patterned to define the conductor pattern. The conductorlayer and, optionally, one or more other layers may be deposited insequence over the surface of the piezoelectric plate. The photoresistmay then be removed, which removes the excess material, leaving theconductor pattern.

At 1140, a front-side dielectric layer may be formed by depositing oneor more layers of dielectric material on the front side of thepiezoelectric plate, over one or more desired conductor patterns of IDTor XBAR devices. The one or more dielectric layers may be depositedusing a conventional deposition technique such as sputtering,evaporation, or chemical vapor deposition. The one or more dielectriclayers may be deposited over the entire surface of the piezoelectricplate, including on top of the conductor pattern. Alternatively, one ormore lithography processes (using photomasks) may be used to limit thedeposition of the dielectric layers to selected areas of thepiezoelectric plate, such as only between the interleaved fingers of theIDTs. Masks may also be used to allow deposition of differentthicknesses of dielectric materials on different portions of thepiezoelectric plate. In some cases, depositing at 1140 includesdepositing a first thickness of at least one dielectric layer over thefront-side surface of selected IDTs, but no dielectric or a secondthickness less than the first thickness of at least one dielectric overthe other IDTs. An alternative is where these dielectric layers are onlybetween the interleaved fingers of the IDTs.

The different thickness of these dielectric layers causes the selectedXBARs to be tuned to different frequencies as compared to the otherXBARs. For example, the resonance frequencies of the XBARs in a filtermay be tuned using different front-side dielectric layer thickness onsome XBARs.

As compared to the admittance of an XBAR with tfd=0 (i.e. an XBARwithout dielectric layers), the admittance of an XBAR with tfd=30 nmdielectric layer reduces the resonant frequency by about 145 MHzcompared to the XBAR without dielectric layers. The admittance of anXBAR with tfd=60 nm dielectric layer reduces the resonant frequency byabout 305 MHz compared to the XBAR without dielectric layers. Theadmittance of an XBAR with tfd=90 nm dielectric layer reduces theresonant frequency by about 475 MHz compared to the XBAR withoutdielectric layers. Importantly, the presence of the dielectric layers ofvarious thicknesses has little or no effect on the piezoelectriccoupling.

In one example, a first thickness of a first dielectric layer depositedbetween the fingers of the IDT of one or more shunt resonators that isgreater than a second thickness of a second dielectric layer depositedbetween the fingers of the IDT of one or more series resonators. Adifference between the first thickness and the second thickness issufficient to set the resonance frequency of the shunt resonator atleast 140 MHz lower than the resonance frequency of the seriesresonator.

In a second variation of the process 1100, one or more cavities areformed in the back-side of the substrate at 1110B. A separate cavity maybe formed for each resonator in a filter device. The one or morecavities may be formed using an anisotropic or orientation-dependent dryor wet etch to open holes through the back-side of the substrate to thepiezoelectric plate. In this case, the resulting resonator devices willhave a cross-section as shown in FIG. 1.

In the second variation of the process 1100, a cap, such as the caps480, 780, 980, 1080, may be attached to the substrate at 1150 to coverand seal the cavities formed at 1110B. The cap may a plate of silicon,glass, or some other material or a plate or film of a polymer material.The cap may be attached to the substrate using any of the previouslydiscussed bonding techniques.

In a third variation of the process 1100, one or more cavities in theform of recesses in the substrate may be formed at 1110C by etching thesubstrate using an etchant introduced through openings in thepiezoelectric plate. A separate cavity may be formed for each resonatorin a filter device. The one or more cavities formed at 1110C will notpenetrate through the substrate, and the resulting resonator deviceswill have a cross-section as shown in FIG. 3A or FIG. 3B.

In all variations of the process 1100, the XBAR filter chip is completedat 1160. Actions that may occur at 1160 include depositing anencapsulation/passivation layer such as SiO₂ or Si₃O₄ over all or aportion of the device; forming bonding pads or solder bumps or othermeans for making connection between the device and external circuitry;and, if necessary, tuning the resonant frequencies of the resonatorswithin the device by adding or removing metal or dielectric materialfrom the front side of the device. At the conclusion of 1160, the XBARfilter chip is ready to be packaged. The process 1100 then ends at 1195.

FIG. 12A, FIG. 12B, and FIG. 12C are, in combination, a flow chart ofprocess 1200 for fabricating a package XBAR filter using a siliconinterposer with TSVs (through silicon vias). While FIG. 12A, FIG. 12B,and FIG. 12C illustrate the process 1200 with an XBAR filter chip withfront-side etched cavities, the process 1200 may also use an XBAR filterchip with back-side etched cavities.

The process 1200 starts at 1205 and ends at 1295 with a completedpackaged XBAR filter. FIG. 12A, FIG. 12B, and FIG. 12C show majorprocess actions, each of which may involve multiple steps. Variousconventional process steps (e.g. surface preparation, cleaning,inspection, baking, annealing, monitoring, testing, etc.) may beperformed before, between, after, and during the steps shown in FIG.12A, FIG. 12B, and FIG. 12C. For each major process action, acorresponding schematic cross-sectional view is provided to illustratethe configuration of the work-in-progress at the conclusion of theaction. Where appropriate, reference designators previously used in FIG.6 are used to identify elements of the work-in-progress.

Referring to FIG. 12A, at 1210, a XBAR filter chip 605 is fabricatedusing, for example, the process 1100 of FIG. 11. The XBAR filter chip605 includes a piezoelectric plate 610 attached to a substrate 620. Thesubstrate 620 may be high resistivity silicon or some other material.Portions of the piezoelectric plate 610 form diaphragms spanningrespective cavities 640 in the substrate 620. A first conductor patternis formed on the surface of the piezoelectric plate 610. The firstconductor pattern includes IDTs with interleaved IDT fingers, such asfingers 630, disposed on the diaphragms.

When the XBAR filter chip 605 has back-side etched cavities (as shown bythe dot-dash lines), a cover 680 is sealed to the back side of thesubstrate 620. While the cover 680 is not shown in subsequentcross-sectional views in FIG. 12A, FIG. 12B, and FIG. 12C, it must beunderstood that all of the actions in the process 1200 are compatiblewith a XBAR filter chip having a cover over back-side etched cavities.

A second conductor pattern is formed on the surface of the piezoelectricplate 610. The second conductor pattern, which may overlay portions ofthe first conductor pattern, may be gold, aluminum, copper or some othermetal. The second conductor pattern includes a continuous metal ring 662around the perimeter of the XBAR filter chip 605. The second conductorpattern also includes pads, such as pad 672, in locations where portionsof the first conductor pattern must be connected to circuitry externalto the packaged XBAR filter.

At 1220, a partially-completed interposer is prepared. Thepartially-completed interposer includes a base 652, which may be highresistivity silicon or some other material. A dielectric layer 654, suchas silicon dioxide, is formed on the surface of the base that will facethe XBAR filter chip. The base 652 may have recesses 655 so that thesurfaces of the base 652 that will face the diaphragms (i.e. the bottomsof the recesses 655) are sufficiently far from the diaphragms. Thedielectric layer 654 may or may not cover the recesses 655. A thirdconductor pattern is formed on top of the dielectric layer 654. Thethird conductor pattern may be the same material as the second conductorpattern. The third conductor pattern includes a continuous metal ring664 around the perimeter of the base 652. The third conductor patternalso includes pads, such as pad 674, in locations where portions of thefirst conductor pattern must be connected to circuitry external to thepackaged XBAR filter. The arrangement of ring 664 and pads 674 of thirdconductor pattern is typically a mirror image of the arrangement of thering 662 and pads 672 of the second conductor pattern.

At 1230, the XBAR filter chip 605 is bonded to the partially-completedinterposer. Specifically, the ring 662 of the second conductor patternis bonded to the ring 664 of the third conductor pattern, forming ahermetic seal around the perimeter of the XBAR filter chip andpartially-completed interposer. Simultaneously, pads, such as pad 672,on the XBAR filter chip are bonded to corresponding pads, such as pad674, on the partially-completed interposer. A preferred method ofbonding the XBAR filter chip to the partially competed interposer isthermocompression bonding, which uses a combination of heat and pressureto make bonds between metallic layers. Other methods, includingultrasonic bonding, and solder or eutectic bonding may be used.

Referring now to FIG. 12B, at 1240, one or both of the substrate 620 andthe XBAR filter chip, and the base 652 of the partially-completedinterposer may be thinned to reduce the overall height of the packageXBAR filter. The substrate 620 and/or the base 652 may be thinned, forexample, by mechanical or chemo-mechanical polishing.

After the optional thinning of one or both of the substrate 620 and thebase 652, through silicon via are formed in a sequence of actions from1250 to 1280.

At 1250, deep reactive ion etching (DRIE) is used to etch holes 1252from the back side (the lower side as shown in FIG. 12B) of the base 652through the base 652 to the dielectric layer 654. The dielectric layer654 is not affected by the DRIE process, so the depth of the etch holeswill be precisely controlled and uniform. The locations of the etchedholes 1252 correspond to the locations of the pads, such as pad 674, ofthe third conductor pattern.

At 1260, a dielectric layer 1262 is deposited over the back side of thebase 652 and the interiors of the holes 1252. The dielectric layer maybe silicon dioxide, silicon nitride, aluminum oxide, or some otherdielectric material. The dielectric layer may be deposited by aconventional process such as evaporation, sputtering, chemical vapordeposition, or some other process.

Referring now to FIG. 12C, at 1270, the oxide layer at the ends of theholes 1252 is etched through a patterned photoresist mask to expose atleast a portion of each contact pad (such as pad 674) of the thirdconductor pattern.

At 1280, a fourth conductor pattern 1256 is formed to create electricconnections from the pads, such as pad 674 of the third conductorpattern, to corresponding pads, such as pad 676 on the exterior surface(the lower surface as shown in FIG. 12C) of the base 652. The fourthconductor pattern may include a primary conductive layer of gold,aluminum, copper or some other highly conductive material. A thin layerof some other metal, such as titanium or nickel may be disposed betweenthe primary conductive layer and the base 652 to improve adhesion. Thestructures including the holes 1252 and the fourth conductor pattern iscommonly referred to as “through silicon vias”. Once the through siliconvias are complete, the process 1200 ends at 1295.

The entire process 1200 may be, and commonly will be, performed on wholewafers. A whole wafer containing multiple XBARs filter chips will bebonded to another wafer containing a corresponding number ofpartially-completed interposers at 1230. The subsequent actions formTSVs for all of the interposers simultaneously. Individual packaged XBARfilters may then be excised by dicing the bonded wafers after action1230.

FIG. 13 is a flow chart of another process 1300 for fabricating apackage XBAR filter using a LTCC interposer. While FIG. 13 illustratesthe process 1300 with an XBAR filter chip with front-side etchedcavities, the process 1300 may also use an XBAR filter chip withback-side etched cavities.

The process 1300 starts at 1305 and ends at 1395 with a completedpackaged XBAR filter. FIG. 13 shows major process actions, each of whichmay involve multiple steps. Various conventional process steps (e.g.surface preparation, cleaning, inspection, baking, annealing,monitoring, testing, etc.) may be performed before, between, after, andduring the steps shown in FIG. 13. For each major process action, acorresponding schematic cross-sectional view is provided to illustratethe configuration of the work-in-progress at the conclusion of theaction. Where appropriate, reference designators previously used in FIG.8 are used to identify elements of the work-in-progress.

At 1310, a XBAR filter chip 805 is fabricated using, for example, theprocess 1100 of FIG. 11. The XBAR filter chip 805 will typically be aportion of a wafer containing multiple XBAR filter chips. The XBARfilter chip 805 includes a piezoelectric plate 810 attached to asubstrate 820. The substrate 820 may be high resistivity silicon or someother material. Portions of the piezoelectric plate 810 form diaphragmsspanning respective cavities 840 in the substrate 820. A first conductorpattern is formed on the surface of the piezoelectric plate 810. Thefirst conductor pattern includes IDTs with interleaved IDT fingers, suchas fingers 830, disposed on the diaphragms.

When the XBAR filter chip 805 has back-side etched cavities (as shown bythe dot-dash lines), a cover or cap 880 is sealed to the back side ofthe substrate 820. While the cover 880 is not shown in subsequentcross-sectional views in FIG. 13, it must be understood that all of theactions in the process 1300 are compatible with a XBAR filter chiphaving a cap over back-side etched cavities.

A second conductor pattern is formed on the surface of the piezoelectricplate 810. The second conductor pattern, which may overlay portions ofthe first conductor pattern, may be gold, aluminum, copper or some othermetal. The second conductor pattern may include pads (not identified) inlocations where portions of the first conductor pattern must beconnected to circuitry external to the packaged XBAR filter. Solderballs or bumps 872 may be formed on the pads to allow the XBAR filterchip 805 to be reflow soldered to an interposer. Alternatively, goldbumps may be formed on the pads to allow the XBAR filter chip 805 to bethermocompression bonded or ultrasonic bonded to an interposer.

At 1320, a LTCC interposer 850 is fabricated by cofiring thin ceramiclayers, some or all of which bear printed conductors. The LTCCinterposer 850 will typically be a portion of a larger panel includingmultiple interposers, An LTCC interposer has at least an upper (as shownin FIG. 13) conductor pattern 874 that includes pads for connections tothe XBAR filter chip and a lower conductor pattern 878 that includespads for connection to circuitry external to the package XBAR filter. Inthe example of FIG. 13, the interposer 850 includes one intermediateconductor layer. An LTCC interposer for an XBAR filter may have morethan three conductor layers. The availability of multiple conductorlayers allows incorporation of passive components, such as inductors,into the interposer.

The LTCC interposer 850 may have recesses 855 to ensure sufficientspacing between the diaphragms and the surfaces of the interposer facingthe diaphragms. Such recess may be formed, for example, by punchingopenings in one or more of the ceramic layers prior to cofiring thelayers of the interposer.

At 1330, the XBAR filter chip 850 is flip-chip bonded to the interposer850. First the XBAR filter chips within a wafer are tested, and goodchips are excised from the wafer. The good chips are then bonded to theLTCC interposer 850 by soldering, thermocompression bonding, ultrasonicbonding, or some other bonding method. The bonding physically attachesthe XBAR filter chip 805 to the interposer 850 and makes electricalconnections between the XBAR filter chip 805 and the interposer 850. Thebonding typically does not make a seal to protect the diaphragms of theXBAR filter chip 805.

At 1340, a polymer cover 860 is formed over the XBAR filter chip 805 toseal the space between the XBAR filter chip 805 and the interposer 850.The cover 850 may be formed by injection molding or casting, forexample. Individual covers may be formed over each XBAR filter chip, ora unitary cover 850 may be formed over the entire LTCC panel. In eithercase, packages XBAR filters may be excised from the panel by, forexample, sawing. The process 1300 then ends at 1395.

FIG. 14 is a flow chart of another process 1400 for fabricating apackage XBAR filter using a wafer-level built up interposer. The process1400 starts at 1405 and ends at 1495 with a completed packaged XBARfilter. FIG. 14 shows major process actions, each of which may involvemultiple steps. Various conventional process steps (e.g. surfacepreparation, cleaning, inspection, baking, annealing, monitoring,testing, etc.) may be performed before, between, after, and during thesteps shown in FIG. 14. For each major process action, a correspondingschematic cross-sectional view is provided to illustrate theconfiguration of the work-in-progress at the conclusion of the action.Where appropriate, reference designators previously used in FIG. 10 areused to identify elements of the work-in-progress.

At 1410, a XBAR filter chip 1005 is fabricated using, for example, theprocess 1100 of FIG. 11. The XBAR filter chip 1005 will typically be aportion of a wafer containing multiple XBAR filter chips. The XBARfilter chip 1005 includes a piezoelectric plate 1010 attached to asubstrate 1020. The substrate 1020 may be high resistivity silicon orsome other material. Portions of the piezoelectric plate 1010 formdiaphragms spanning respective cavities 1040 in the substrate 1020. Afirst conductor pattern is formed on the surface of the piezoelectricplate 1010. The first conductor pattern includes IDTs with interleavedIDT fingers, such as fingers 1030, disposed on the diaphragms.

The subsequent actions in the process require liquid materials, such assolvents, photoresist, or photopolymerizable monomers, to be applied tothe front side of the piezoelectric plate 1010 after the cavities 1040have been etched. The process 1400 is not suitable for XBAR filter chipswith front-side etched cavities because the liquid materials may passinto the cavities through the etch holes in the diaphragms. Thus, theXBAR filter chip 1005 has back-side etched cavities with a cover 1080sealed to the back side of the substrate 1020.

At 1420, walls 1052 are formed on the piezoelectric plate 1010. Thewalls 1052 may be formed with openings over the XBAR diaphragms andopenings where electrical connections to the XBAR filter chip will bemade in a subsequent process action. The walls 1052 may be formed, forexample, by coating the piezoelectric plate 1010 with aphotopolymerizable material and then exposing the photopolymerizablematerial through a suitable mask. Depending on the required thickness ofthe walls, multiple layers of material may be coated and patterned insuccession.

A 1430, a cover layer 1054 is applied over the walls 1052. The coverlayer 1054 may be applied, for example, as a continuous film bonded tothe walls 1052 by an adhesive. The cover layer 1054 spans the openingsin the walls 1052 over the XBAR diaphragms, forming an enclosed cavity1055 over each diaphragm. The cover layer is pattered to form openingswhere electrical connections to the XBAR filter chip will be made in asubsequent process action.

At 1440, a conductor pattern 1070 is formed. The conductor pattern 1070includes pads 1072 on the external surface of the cover layer 1054 forconnection to circuitry external to the packaged XBAR filter. Theconductor pattern 1070 connects the pads 1072 to connection points 1074on the XBAR filer chip 1005. The conductor pattern 1070 may be aluminum,copper, gold, or a combination of materials deposited and patternedusing conventional techniques. Once the conductor pattern is formed, theprocess 1400 ends at 1495.

The entire process 1400 may be, and commonly will be, performed on wholewafers. Individual packaged XBAR filters may then be excised by sawingthrough the bonded wafers after the conductor pattern is formed at 1440.

CLOSING COMMENTS

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. An acoustic resonator device comprising: an acousticresonator chip comprising: a substrate having front and back surfaces,at least first and second cavities formed in the front surface; apiezoelectric plate having front and back surfaces, the back surfaceattached to the front surface of the substrate except for portions ofthe piezoelectric plate that form at least first and second diaphragmsspanning the first and second cavities, respectively; and a firstconductor pattern formed as one or more conductor layers on the frontsurface of the piezoelectric plate, the first conductor patterncomprising: at least first and second interdigitated transducers (IDTs),interleaved fingers of the first and second IDTs on the first and seconddiaphragms, respectively, and a first plurality of contact pads; aninterposer having front and back surfaces, the interposer comprising: asecond plurality of contact pads on the interposer back surface; and aplurality of conductive balls bonding each of the contact pads of thefirst plurality of contact pads to respective contact pads of the secondplurality of contact pads.
 2. The acoustic resonator device of claim 1,wherein the interposer further comprises conductors and conductive vias,and wherein the conductive vias and conductors connect the secondplurality of contact pads to a third plurality of contact pads on theinterposer front surface.
 3. The acoustic resonator device of claim 2,wherein the conductors form passive circuit components comprising atleast one inductor.
 4. The acoustic resonator device of claim 1, whereinthe interposer comprises a low temperature co-fired ceramic circuitcard.
 5. The acoustic resonator device of claim 1, wherein theinterposer comprises a printed circuit board.
 6. The acoustic resonatordevice of claim 1, further comprising a molded cover forming a sealextending around and connecting an outer perimeter of the acousticresonator chip to a perimeter of the interposer.
 7. The acousticresonator device of claim 6, wherein the molded cover is a polymer moldover the acoustic resonator chip that seals side surfaces of theacoustic resonator chip to a perimeter of the interposer front surface.8. The acoustic resonator device of claim 1, wherein the plurality ofconductive balls are one of: solder bonded solder balls, orthermocompression bonded gold balls, or ultrasonic bonded gold balls. 9.The acoustic resonator device of claim 1, the interposer furthercomprising: at least first and second recesses in the interposer backsurface facing the at least first and second diaphragms, respectively.10. The acoustic resonator device of claim 9, wherein a distance from abottom of each recess to the respective diaphragm is greater than orequal to 15 microns and less than or equal to 100 microns.
 11. Theacoustic resonator device of claim 1, wherein the at least first andsecond IDTs and the piezoelectric plate are configured such thatrespective radio frequency signals applied to the IDTs excite shearprimary acoustic modes within the respective diaphragms.
 12. Theacoustic resonator device of claim 1, wherein a thickness between thefront and back surfaces of the piezoelectric plate is greater than orequal to 200 nm and less than or equal to 1000 nm; a respective pitch offingers of each of the first and second IDTs is greater than or equal to2 times the thickness of the piezoelectric plate and less than or equalto 20 times the thickness of the piezoelectric plate; and the fingers ofeach of the first and second IDTs have a respective width, and therespective pitch is greater than or equal to 2 times the respectivewidth and less than or equal to 20 times the respective width.
 13. Theacoustic resonator device of claim 1, wherein a first thickness of afirst dielectric layer deposited between the fingers of the first IDT isgreater than a second thickness of a second dielectric layer depositedbetween the fingers of the second IDT.
 14. An acoustic resonator devicecomprising: an acoustic resonator chip comprising: a substrate havingfront and back surfaces, at least two cavities formed in the frontsurface; a piezoelectric plate having front and back surfaces, the backsurface attached to the front surface of the substrate except forportions of the piezoelectric plate that form at least two diaphragmsspanning the at least two cavities; and a first conductor pattern formedas one or more conductor layers on the front surface of thepiezoelectric plate, the first conductor pattern comprising: at leasttwo interdigitated transducers (IDTs), interleaved fingers of each ofthe IDTs on one of the diaphragms; and and a first plurality of contactpads; an interposer comprising a second plurality of contact pads on aninterposer back surface; and a plurality of conductive balls directlybonding each contact pad of the first plurality of contact pads to arespective contact pad of the second plurality of contact pads, whereinthe piezoelectric plate and the at least two IDTs are configured suchthat a respective radio frequency signal applied to each IDT excites abulk shear acoustic wave within the respective diaphragm.
 15. Theacoustic resonator device of claim 14, wherein the interposer furthercomprises conductors and conductive vias, and wherein the conductivevias and conductors connect the second plurality of contact pads to athird plurality of contact pads formed on an interposer front surface.16. The acoustic resonator device of claim 15, wherein the conductorsform passive circuit components comprising at least one inductor. 17.The acoustic resonator device of claim 14, wherein the interposercomprises a low temperature co-fired ceramic circuit card.
 18. Theacoustic resonator device of claim 14, wherein the interposer comprisesa printed circuit board.
 19. The acoustic resonator device of claim 14,further comprising a molded cover forming a seal extending around andconnecting an outer perimeter of the acoustic resonator chip to aperimeter of the interposer, wherein the molded cover is a polymer moldover the chip that seals side surfaces of the chip to a perimeter of aninterposer front surface.
 20. A filter device, comprising: an acousticresonator chip comprising: a substrate; a piezoelectric plate having aback surface attached to the substrate; and a conductor pattern formedon a front surface of the piezoelectric plate, the conductor patterncomprising: a plurality of interdigital transducers (IDTs) of arespective plurality of acoustic resonators, interleaved fingers of eachof the plurality of IDTs on respective portions of the piezoelectricplate suspended over one or more cavities formed in the substrate;conductors to connect the plurality of acoustic resonators in a ladderfilter circuit; and a first plurality of contact pads; an interposercomprising a second plurality of contact pads on an interposer backsurface; and a plurality of conductive balls bonding each of the contactpads of the first plurality of contact pads to respective contact padsof the second plurality of contact pads.
 21. The filter device of claim20, wherein the plurality of resonators comprises one or more shuntresonators and one or more series resonator, and the acoustic resonatorchip further comprises: a first dielectric layer having a firstthickness between the fingers of each IDT of the one or more shuntresonators; and a second dielectric layer having a second thickness,less then the first thickness, between the fingers of each IDT of theone or more series resonators.
 22. The filter device of claim 20,wherein the plurality of IDTs and the piezoelectric plate are configuredsuch that respective radio frequency signals applied to the IDTs exciteshear primary acoustic modes within the respective diaphragms.
 23. Thefilter resonator device of claim 20, wherein the interposer furthercomprises conductors and conductive vias, and wherein the conductivevias and conductors connect the second plurality of contact pads to athird plurality of contact pads formed on an interposer front surface.24. The filter device of claim 23, wherein the conductors form passivecircuit components comprising at least one inductor.
 25. The filterdevice of claim 20, wherein the interposer comprises a low temperatureco-fired ceramic circuit card.
 26. The filter device of claim 20,wherein the interposer comprises a printed circuit board.
 27. The filterdevice of claim 20, further comprising a molded cover forming a sealextending around and connecting an outer perimeter of the acousticresonator chip to a perimeter of the interposer, wherein the moldedcover is a polymer mold over the chip that seals side surfaces of thechip to a perimeter of the interposer front surface.